In a heat pump and refrigeration cycle, refrigerant alternately absorbs and rejects thermal energy as it circulates through the system and is compressed, condensed, expanded, and evaporated. In particular, a liquid refrigerant flows from a condenser, through an expansion device (e.g., expansion valve) and into an evaporator. As the refrigerant flows through the expansion device and evaporator, the pressure of the refrigerant decreases, the refrigerant phase changes into a gas, and the refrigerant absorbs thermal energy. From the evaporator, the gaseous refrigerant proceeds to a compressor, and then back to the condenser. As the refrigerant flows through the compressor and condenser, the pressure of the refrigerant is increased, the refrigerant phase changes back into a liquid, and the refrigerant gives up thermal energy. The process is implemented to emit thermal energy into a space (e.g., to heat a house) or to remove thermal energy from a space (e.g., to air condition a house).
A heat recovery ventilator (HRV) can help make mechanical ventilation more cost effective by reclaiming energy from exhaust airflows. HRVs use heat exchangers to heat or cool incoming fresh air, recapturing up to 80 percent of the conditioned temperatures that would otherwise be lost. Models that exchange moisture between the two air streams are referred to as Energy Recovery Ventilators (ERVs). ERVs are especially recommended in climates where cooling loads place strong demands on heating, ventilation, and air conditioning (HVAC) systems. In operation, ERVs transfer moisture from the humid air stream (incoming outdoor air in the summer) to the exhaust air stream. Efficient use of ERVs is not a trivial task and differs according to the installation environment.